PROLOCULAR PHASE.-The prolocular phase consists of the first chamber formed in the ontogenetic series. Prolocular stage: note the absence of pores in the wall of the proloculus (P), but the presence of pores in the deuteroconch (D) and subsequent chambers (a).
The opening position in the initial whorl is nearly equatorial, the chamber surfaces are smooth, and the chamber morphology is spherical to slightly compressed. Pore diameters typically vary between 0.5 and 1.0 Lim, pore densities and pore densities reach a maximum of approx. 10 pores/625 Lim2 in the last vortex chambers (Figure 14). It is much more common in the middle to high latitudes of both hemispheres than in the low latitudes and has been shown to date back to the late Campanian of the southern South Atlantic (Huber, 1991a).
The increase in chamber size in the terminal whorl is relatively slow, as indicated by the low size ratio of penultimate/penultimate chamber diameters. Penultimate whorl: The mean of 5.15 chambers in the penultimate whorl is slightly less than the number of chambers in the terminal whorl, which averages 5.35 (Table 4). The shape of the chamber does not change in the penultimate whorl, and the surface ornaments remain smooth to very finely pustulate.
In adult chambers, the cylindrical part of the pore is bounded on the inner wall by a wide funnel-shaped opening (Figure 6a).
EXTERNAL OBSERVATIONS OF THE TEST.— The test is coiled into a moderately low trochospira forming a square to subcircular, strongly spatulate outline with a rounded axial margin. Due to the smaller size of the prolocus, the number of chambers in the initial whorl is greater, averaging 5.42 with a range of 4.75 to 6.00. Fewer chambers appear in the final whorl due to the tendency for a more involute (tighter) coiling pattern.
Belford placed these species in Whiteinella, but indicated that the type description of that genus made no reference to surface ornamentation of the test (Pessagno, 1967). 1983) subsequently described a new genus, Costellagerina, to accommodate Belford's species, and they designated C. bulbosa as the type species. pilula in the family Rotaliporidae and subfamily Hedbergellinae because of the extraumbilical-umbilical opening, absence of tegilla and presence of portici. 1984), apparently unaware of the work of Petters et al. 1983), placed the meridionally costellate species of Belford in Rugoglobigerina and synonymized the four-chambered morphotype (= R. bulbosa from Belford, 1960) under R. Although the log-linear growth curves and the extraumbilical position of the opening have a closer affinity with the hedbergellid lineage, C.
The latter genus is easier to separate from Archaeoglobigerina because of the extraumbilical position of the primary opening.
OBSERVATIONS OF THE TEST OUTSIDE.-The test is trochospiral with a moderately high dorsocomcity, the equatorial periphery is deeply lobed, and the axial periphery is broadly rounded. The umbilicus is deep, narrow to wide, accounting for an average of 28% of the maximum test diameter. On the interior of adult chambers, a narrow funnel-shaped depression borders the cylindrical portion of the pore openings (Figure la).
However, a conventional approach to taxonomic classification would categorize end-piece phenotypes (e.g., Figure 2a-o", i-l) into different genera due to differences in opening positions and rate of increase in chamber size in final whorls. the third cause is the accumulation of smaller foraminiferal size classes with flocculent material that is sinking through the water column and settling on the seafloor (Hemleben et al., 1988) Note that adult morphotypes Leg 71 and Leg 113 compare closely in their mean values, but the mean value of the neanic specimens decreases especially after the eighth chamber.
Perhaps some of the size and morphological variation demonstrated by micromorphic and gerontic populations of A.
Heterochrony: An alternative interpretation of the taxonomy of the micromorph populations is that they represent a distinct new species that evolved through the heterochronic process of paedomorphosis, where the juvenile characteristics of ancestral A. However, independent evidence for this view is lacking, as the stratigraphic extent and area distribution of the micromorph and gerontic populations are the same. However, some of the Seymour Island forms differ in their external morphology in that they have a faint development of meridional costellae and a tegillum covering the umbilical region (Plate 9: . Figures 9,13), showing a strong affinity with R proposal.
EXTERNAL OBSERVATIONS OF THE TEST.—The test is coiled into a moderately low trochospire and forms a quadrate to subcircular outline with a broadly rounded axial periphery and. The correlation between the diameter of the proloculus and the initial whorl is quite low, with a correlation coefficient of 0.67. The morphology of the chamber within the initial whorl is globular and the apertural position remains extraumbilical.
Apertural position shifts from extraumbilical to umbilical-extraumbilical, chamber morphology is globular, and surface ornamentation becomes more densely pustular within this part of ontogenesis.
OBSERVATIONS OF THE TEST EXTERIOR.-The test is coiled into a low trochospire forming a moderately lobed, subcircular equatorial outline. The umbilicus is moderately broad and shallow, comprising on average 29% of the maximum test diameter. Morphological changes occurring in the penultimate vertebra include movement of the opening to an umbilical extraumbilical position and increased axial compression of the chambers, leading to a reniform cross-sectional morphology.
By the fifth or sixth chamber, pores have spread outward from the inner spiral suture of the chambers (Figure 30a,c), but the axial circumference remains poreless throughout ontogeny (Figure 30b). Banner and Blow further noted that a fragile tegillum is preserved on several of the d'Orbigny syntypes, but not on the lectotype. FIGURE 29—Arithmetic and logarithmic plots of the chamber-by-chamber increase in diameter chamber area (in um2) of Archaeoglobigerina cretacea.
Note that pores are not present in the proloculi and are initially concentrated along the spiral sutures in both species, b is a transverse section of the poreless peripheral margin of the twelfth chamber of A. cretacea.
OBSERVATIONS FROM THE OUTSIDE OF THE TEST. The test is coiled into a moderate to high steeple, even to unevenly biconvex, with the spiral side often more convex than the umbilical side. With an average test width of 186 |im, the average width/diameter ratio of 0.69 is the highest of the species measured in this study (Table 4). The umbilicus is generally small and deep, accounting for an average of 19% of the total test diameter.
The opening is the umbilical cord, but is usually obscured by a broad portico or bulging extension of the posterior chamber. Due to considerable variation in proloculus sizes, there is also a wide range of initial whorl diameters, with most between 50-83 mm and two between 113-117 |im. No pores are observed in the first three chambers, and there are very few near the spiral suture from the sixth to seventh chambers, where the pores become more evenly distributed throughout the rest of the ontogenetic sequence (Fig. 16/).
Its absence from pre-Maastrichtian sediments of Leg 114 Hole 700C may be an artifact of poor preservation of Campanian sediments (Huber, 1991a).
COMMENTS. At the outset of this study, it was hoped that serial dissection of high-latitude morphotypes considered rugoglobigerinids due to the presence of weak, meridionally aligned costellae and tegilla (e.g., Plate 5: Figures 1-4, Plate 9: Figures 1-4, Plate 9: Figures 1-4). 9-15) would reveal distinctive ontogenetic features that they associate with serially dissected topotypes of Rugoglobigerina. Unfortunately, this study was unable to resolve the taxonomic uncertainties among this group. The umbilicus is moderately narrow and deep, comprising an average of 21% and a range of 9% to 30% of the maximum test diameter.
The wall is generally medioperforate, but may be macroperforate in the last whorl chambers. Apertural position changes from extraumbilical in the early part of the penultimate whorl to umbilical in the later chambers, and surface ornamentation grows from hispid to coarsely pustulose or costellate by the penultimate whorl chamber. Ontogenetic growth curves: The rate of increase in chamber size is log-linear throughout most of the ontogeny of R.
Therefore, the ontogenetic data obtained provide additional criteria for comparison with Costellagerina pilula, which is the type species of the only other meridional costellate genus of the Late Cretaceous (Tables 2, 3).
The surface ornamentation of neanic forms consists of faint costellae or randomly placed pustules, and the end chambers of the whorl grow quite quickly in size, giving it a "big head". Therefore, the surface water habitat exerted some control over the architectural features developed on the chamber surface. The criteria used to distinguish transitions from one ontogenetic stage to another within the Cretaceous taxa in this study are based on qualitative observations, such as changes in aperture position, chamber surface ornamentation, umbilical width and depth, and chamber morphology ( Figure 9). and quantitative data, including sudden changes in the rate of increase in room size (Figure 36).
An additional method for characterizing growth transitions is to measure ontogenetic changes in the average porosity of the shell (Figure 37). Additional studies of the same species from different environments will provide a more accurate picture of the variance in porosity values per chamber. Abrupt increases in shell porosity that occur within the first ten chambers of the mean plots (Figure 37) are used to identify the transition from juvenile to neonic stages.
Thus, a transition from the juvenile to the adult stage is inferred between the tenth and twelfth chambers based only on the porosity data for H.
The onset of the adult stage is identified by the appearance of meridionally aligned pustules on the chamber surface. Maximum porosity is reached in the eleventh chamber, which marks the beginning of the adult stage for most specimens. Maximum porosity is reached around the eleventh to twelfth chamber, which marks the beginning of the adult stage.
Other ontogenetic curves have fairly uniform rates of increase in chamber size after the onset of the neanic phase. Most specimens show a uniform distribution of pores until the beginning of the penultimate whorl in mature specimens. Maestrichtian Planktonic Foraminiferal Biostratigraphy of the Maud Rise (Weddell Sea, Antarctica): ODP Leg 113 Holes 689B and 690C.
FIGURES 5-7.—Serial section of individual specimen (236 grill): 5, specimen with cut outer ring; 6, same specimen with all ventricles dissected; 7, increase in initial vortex. Note the similarity of the initial whorl in this morphotype to the initial whorls of five-chambered morphotypes. FIGURES 7,8.—Complete serial dissection of a nean specimen showing the entire specimen and magnification of the initial whorl (219 um).
